This invention relates to a circuit arrangement for igniting a high-pressure discharge lamp, provided with input terminals for connection to a supply source; a pulse generating circuit having a natural frequency and provided with a voltage-dependent breakdown element; a pulse transformer; and an electrical connection between a secondary winding of the pulse transformer and lamp connection points.
Conventional starter circuits, such as disclosed in U.S. Pat. No. 5,087,859 are well known for starting a high-pressure discharge lamp within a projection TV system. Special requirements are imposed on a lamp suitable for such an application as to its dimensions and luminous efficacy. An ignition pulse of more than 10 kV, often even more than 20 kV, can be required for reliable lamp ignition. An ignition circuit is also required to quickly reignite an extinguished lamp which still may be hot. Such hot re-ignition requires a re-ignition pulse of an amplitude corresponding to that of the ignition pulse.
The voltage pulse in the pulse transformer is applied to the lamp connection points directly from the second winding. Ignition pulse generation takes place in the known circuit arrangement by operating as a tuned LC circuit near its resonance whereby the voltage across the capacitive element of the circuit is increased until the breakdown voltage of the breakdown element is reached. For efficient operation, the supply source is commuted with a comparatively high frequency in the tens of kHz during the ignition pulse generating process. The natural frequency of the pulse generating circuit is the frequency of the tuned circuit formed by the primary winding of the pulse transformer and the resonant capacitive element of a voltage-raising network. After the lamp has ignited and a stable arc discharge has developed, the commutation frequency of the supply voltage switches to a comparatively low value. The lamp behaviour from breakdown up to the moment a stable discharge is achieved is called the ignition phase. The choice of a high commutation frequency in the ignition phase renders possible the use of comparatively small components, while the generation of a sufficiently high voltage will take up comparatively little time. This does presuppose, however, that the supply source circuit is capable of changing the commutation frequency in dependence on the state of the lamp.
Takeover by the supply source of the supply of current to the lamp occurs immediately after lamp breakdown. The take-over behaviour of the starting circuit often exhibits an undesirable gradient, that is the supply of current is initiated too slowly, so that the ionization in the lamp is lost and a new ignition pulse is necessary.